The invention provides an improved apparatus for rapidly thermally stabilizing organic fibers which undergo exothermic reaction during heat treatment in an oxidizing atmosphere, particularly acrylic fibers, whereby the heat produced during oxidative stabilization of the fibers is removed via radiative heat transfer.
The thermal oxidative stabilization of a bundle of organic fibers which undergo exothermic reaction during treatment thereof historically has required a treatment of relatively long duration (e.g., elapsed time of at least about 2 to 24 hours), to obtain a degree of stability at which the fiber bundle is non-burning when subjected to an ordinary match flame and will withstand carbonization temperatures without loss of its fibrous structure. The exothermic oxidation reaction causes the temperature of the fiber bundle to rise above that of the oxidation chamber. Such an excess temperature rise can lead to disintegration of the fibers.
The capacity of a given oxidative stabilization system for this type of fibers is limited by this behavior. The oxidation rate can be increased by raising the treatment temperature, but only to the limit imposed by the rate of heat evolution from the fibers being treated, which also rises with increasing temperature. The fiber throughput can be increased by increasing the packing density or number of filaments passed through the oxidizing chamber. This throughput is limited by the increasing quantity of heat given off during fiber stabilization and by the efficiency with which the system can remove the exothermic heat from the fibers.
A combination of high oxidation rate and high packing density can only be achieved if the fiber oxidation apparatus is constructed to provide for efficient removal of heat from the reacting fiber. Only recently has it been discovered that in a typical oxidation oven, radiation is the principal mechanism of heat transfer between the fiber and oxidizing chamber walls, rather than convection, as has been assumed. For this reason, organic fiber oxidation equipment built to date has been designed to effect heat transfer by convection.
The walls of a conventional chamber for the thermal oxidation of organic fibers are fabricated of thick (e.g., 18 gauge) carbon steel or stainless steel or the like and have a smooth inner surface. Aditionally, the walls are usually insulated. Such walls possess low radiation absorption capacity, low thermal conductance and thus poor heat transfer characteristics. Further, in cases where more than one layer of fibers pass through the apparatus, such layers are not divided by physical barriers, resulting in a situation in which radiant heat is exchanged between fiber layers, rather than being removed from the fibers in a direct manner.